How to copy NLA strip without creating a new track?
Yes, maybe I miss something but it doesn't seem logical, maybe it's worth a report to Blender developers? That said, you can either:


Duplicate one or several times your strip, it will create one new track but you can delete it later (put your cursor over the track's name and press X).
Put your cursor where you want to create a copy of your strip and press ShitA > Strip Name, but it won't keep all the edits you could have made in the NLA "N" panel.